The present invention relates to a screen printing machine wherein a film of solder is formed on a board by moving a squeegee via a screen.
It is a recent pressing demand to enhance the accuracy of a screen printing machine because more and more higher-density and higher-quality circuit boards are required. It is also increasingly demanded for the screen printing machine to be ready to switch the printing mode, rise and start immediately after the maintenance work, with exerting conveniences of manipulation. By way of example, there is disclosed a screen printing machine in Japanese Laid-Open Patent Publication No.5-193104 (193104/1993) which is adapted to automatically control the printing thickness. The prior art machine changes a squeegeeing angle in accordance with the output data from a pulse motor and a displacement sensor measuring the printing film thickness, thereby maintaining a target film thickness.
A conventional screen printing machine of the type referred to above, that is, which automatically controls the film thickness will be depicted with reference to the corresponding drawings. Referring to FIG. 8, element 41 is a pulse motor; element 42 is an encoder; element 43 is an air cylinder; element 44 is a squeegee; element 45 is a screen; element 46 is a displacement sensor; and element 47 is a CPU.
The screen printing machine comprising the above components operates in the following manner. As indicated in the flowchart of FIGS. 9 and 10, first, at step S61 cream solder is printed by the squeegee 44 and the air cylinder 43 which maintains the pressing pressure by the squeegee 44 constant. The thickness of the printed cream solder is detected by the displacement sensor 46 at step S62. When the measuring result is output to the CPU 47, the CPU 47 uses a difference of the measured thickness between the thickness of the printed cream solder and a target film thickness as a to-be-fed back angle for the squeegee, whereby the pulse motor 41 and the encoder 42 are variably controlled to obtain the target film thickness at steps S63 and S64. It is to be noted here that the CPU 47 operates the film thickness as proportional to the angle of the squeegee, as shown in FIG. 8.
In the constitution of the conventional screen printing machine as above, as understood from FIG. 11, even when the angle of the squeegee 44 is increased from .alpha.1 to .alpha.2 so as to attain a larger rolling angle, the pressing amount to the screen 45 is so much as to wipe the cream solder off, resulting in the reduced film thickness.
FIGS. 12, 13, and 14 respectively illustrate cases when the squeegee 44 is changed, when the squeegeeing length is changed from L1 to L2 and when the screen thickness is changed from t1 to t2, or vice versa. It is impossible in any of the arrangements in FIGS. 12-14 to control the film thickness to be optimum merely by keeping the squeegeeing angle constant so as to thereby maintain the optimum rolling angle. Moreover, the prior art machine cannot adjust the film thickness for such a squeegee, e.g., a sword-shaped squeegee or a prism squeegee that is pressed in touch with the screen 45 approximately at right angles and not inclined to the screen 45.